Within the present application the term semiconductor disk gain medium refers to the gain medium known in the field of Semiconductor Disk Lasers (SDLs). It is noted that SDLs are also known in the art as Vertical External Cavity Emitting Lasers (VECSELs) or Optically Pumped Semiconductor Lasers (OPSLs). Therefore the term semiconductor disk gain medium when used throughout the present description is used also to refer to the gain medium of each of these systems.
The term “ultra short” pulses as used within the following description refers to pulses having a duration from about 100 picoseconds (ps) down to a few femtoseconds (fs).
An optical amplifier is a device that amplifies an input optical field directly, without the need to first convert the input optical field into an electrical signal. A number of different forms of optical amplifier are known in the art, some of which are described briefly below.
One form of optical amplifier known in the art is a Doped Fibre Amplifier (DFA). These optical amplifiers employ a doped optical fibre as a gain medium to amplify the input optical field. In practice, the optical field to be amplified and a pump field are multiplexed into the doped fibre, and the optical field is then amplified through interaction with the doping ions. The most common example of DFA is an Erbium Doped Fibre Amplifier (EDFA) although Thulium, Praseodymium and Ytterbium doped fibre amplifiers have also been successfully demonstrated. DFAs are however relatively expensive systems to produce. Furthermore, due to the design restrictions it is not possible to employ a DFA to amplify an input optical field having a wavelength less than 1 μm.
Alternative optical amplifiers known in the art are those based on solid state crystals. Optical amplifiers based on Ti Sapphire crystals pumped with a 514 to 532 nm wavelength source are capable of amplifying an input optical field having a wavelength less than 1 μm. Two different designs exist for such amplifiers: regenerative amplifiers and multi-pass amplifiers. In a regenerative amplifier the input optical field is amplified within a resonator. However, unlike normal laser resonators that comprise a partially reflective mirror that functions as an output coupler, the amplifier resonator comprises high-speed optical switches that insert the optical field into the resonator and then extract the pulse out of the resonator at the moment when it has been appropriately amplified. In the multi-pass amplifier design, there are no optical switches. Instead, mirrors guide the input optical field a fixed number of times (two or more) through the solid state crystal with slightly different directions. These types of optical amplifiers generally require relatively large footprints and are also relatively expensive to produce.
Optical Parametric Amplifiers (OPAs) employ the nonlinear properties of crystal materials to provide the means of amplification to an input optical field. Here, the input optical field propagates through the nonlinear crystal together with a pump field of shorter wavelength. Photons from pump field are then converted into (lower-energy) signal photons (which act to amplify the input optical field) and the same number of so-called idler photons. The photon energy of the idler wave is the difference between the photon energies of the pump and signal waves. As the pump energy is fully converted into energy of signal and idler beams, the crystal material is not heated by this process. OPAs are however complex devices, that require relatively large footprints and are relatively expensive to produce.
A range of semiconductor based optical amplifiers (SOAs) have also been developed. Examples of semiconductor optical amplifiers are those made from group III-V compound semiconductors e.g. GaAs/AlGaAs, InP/InGaAs, InP/InGaAsP and InP/InAlGaAs Other direct band gap semiconductors can equally well be employed, such as group II-VI compound semiconductors.
More recently, it has proved advantageous for such gain mediums to be provided with a tapered profile. For example GaAs based tapered amplifiers have been used for the amplification of input optical fields (15 mW and 30 mW and wavelengths between 755 nm and 1064 nm) to provide nearly diffraction limited power values of up to 2 W.
A further addition to the SOA family is the vertical-cavity SOA (VCSOA). These devices are similar in structure to, and share many features with, vertical-cavity surface-emitting lasers (VCSELs). The major difference when comparing VCSOAs and VCSELs is the reduced mirror reflectivity employed within the amplifier cavity. Given their vertical-cavity geometry, VCSOAs have the advantage that they are resonant cavity optical amplifiers that operate with the input and output optical fields entering and exiting normal to the wafer surface.
The above described SOAs are all relatively compact and cheaper to produce when compared with the previously described DFAs, solid state crystal amplifiers and OPAs. However, with SOAs it can prove difficult to couple the signal field into the gain medium. In addition, the use of electrical pumping and the inherent nonlinearity of the semiconductor materials can lead to pulse distortion and even to the breakup of the output field pulses. As a result the SOAs known in the art are not found to provide long term stability characteristics on the amplified output field and often generate outputs that are of low beam quality.
It is therefore an object of an aspect of the present invention to obviate or at least mitigate the foregoing disadvantages of the optical amplifiers known in the art.